The aim of the study was to analyze the usefulness and side effects of treatment with interferon beta 1B (Betaferon) in patients with the relapsing-remitting form of multiple sclerosis (RRMS). The study included 32 RRMS patients that had completed two-year therapy with interferon beta 1B or were still receiving this therapy. Every six months, patients were clinically evaluated and scored by the Expanded Disability Status Scale (EDSS). Two-year therapy was completed by 11 (34.3%) of 32 RRMS patients. Relapse was verified in 4 (36.36%) patients. The mean EDSS score was 2.45 +/- 1.03 at the beginning of therapy and 2.54 +/- 0.98 after two-year therapy; the difference was not statistically significant. In 2 (6.25%) patients on therapy for 18 months there was no relapse, and the mean EDSS was 1.75 +/- 0.35 (both at therapy introduction and at 18 months). Five (15.62%) patients were on therapy for one year. The mean EDSS was 1.6 +/- 1.08 at the beginning of therapy and 1.5 +/- 0.70 at one year. One patient experienced relapse. Two patients were on therapy for six months. They had no relapses with the same EDSS at six months as at therapy introduction (2.0). At the beginning of 2008, another 12 patients started therapy with interferon beta 1B. In conclusion, our experience with two-year interferon beta-1B therapy for RRMS is favorable, with a relatively low rate of relapses (36.36%) and without significant worsening on EDSS. The medication side effects were mild and transient.